Conventionally, in optical communication and optical measurement fields, there has been broadly used a waveguide modulator in which an optical waveguide and modulation electrodes are formed on a substrate having electro-optic effect. In such an optical modulator, there are demanded multi functionalization and compact in size. In addition, as shown in FIG. 1, there has been used a method in which a connection substrate 4 and a terminal substrate 9 are disposed around the optical modulation element 1, and are integrally mounted in a casing 10, and thus an optical modulator module is formed.
In an example of the optical modulator shown in FIG. 1, an optical modulation element 1 is formed of an optical waveguide (not shown in the drawings) and a modulation electrode on a substrate made of LiNbO3 and the like having electro-optic effect. The modulation electrode is constituted of a signal electrode 2, a ground electrode (not shown in the drawings), and the like. To the optical modulation element 1, an optical fiber 3 for emitting and receiving a light wave is connected.
In addition, a connection substrate 4 including a functional element 8 such as an amplifier and a terminal substrate 5 including terminator 9 are disposed around the optical modulation element 1. The connection substrate 4 and the terminal substrate 5 are encased in a casing 10 with the optical modulation element 1, and constitute the optical modulator module.
Hereinafter, a method for driving the optical modulator will be described. A microwave signal generated from a modulation signal source 6 is inputted to a GPO connector 7 serving as an input terminator of the casing 10, and is propagated from the connector to a signal input end 11 of the connection substrate 4 shown in FIG. 1(b).
In the connection substrate 4, modulation signals of an amplifier, a distributor, a phase shifter, and the like are outputted to a signal output end 12 through the functional element 8 for converting the modulation signal into various states. In addition, the connection substrate 4 is not limited to include the functional element 8, and as disclosed in Patent Document 1, for example, the connection substrate may have only a coplanar line path.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-233043
A signal output end 12 of the connection substrate is wire bonded to an electrode pad of the signal electrode 2 of the optical modulation element, and the modulation signal output from the connection substrate 4 is propagated to the signal electrode 2. Then, a light wave propagated in the optical waveguide of the optical modulation element is optically modulated by the modulation signal propagated to the signal electrode 2.
On a terminal portion of the signal electrode 2, the other electrode pad is provided, the electrode pad is wire bonded to the end of the signal line of the terminal substrate in the same manner as described above. Hence, the modulation signal is further propagated from the signal electrode 2 to the terminal substrate 5, and is absorbed by the terminator 9 provided in the terminal substrate.
Meanwhile, when the optical modulation element modulates intensity of light, a modulation curve D (a curve illustrating light intensity variation I of the modulated light wave with respect to a voltage V applied to the optical modulation element) of the optical modulation element 1 is shown in FIG. 2. Hence, particularly, the modulation signal applied to the signal electrode 2 of the optical modulation element 1 is set to vary in the range from the top point at which light output is at the maximum to the bottom point at which light output is at the minimum as represented by the reference sign a shown in FIG. 2. Variation in light output at the time of applying the modulation signal a is represented by the reference sign A.
When the modulation signal is smaller than a predetermined amplitude value as shown in FIG. 2 (in a case of a modulation signal b), the light output thereof varies as the reference sign B, and becomes smaller than a predetermined amplitude value of light output. Therefore, S/N ratio deteriorates. In addition, when the modulation signal is larger than the predetermined amplitude value as shown in FIG. 2 (in a case of a modulation signal c), the light output thereof varies as the reference sign C, and becomes smaller than the predetermined amplitude value of light output. Therefore, S/N ratio deteriorates, and simultaneously a light output waveform thereof is distorted.
As described above, to stably maintain modulation characteristics of the optical modulation element, it is necessary to constantly maintain at a predetermined value the voltage amplitude value of the modulation signal applied to the optical modulation element.
However, generally, a modulation signal for driving the optical modulation element is about 5V, while the modulation signal output from the modulation signal source 6 is about 0.3V. Thus, there has been used a method of amplifying modulation signal by use of an amplifier. Accordingly, since amplification ratio of the amplifier varies with temperature variation in the optical modulator module or temperature difference between the inside and outside of the module, there has been caused a problem that the amplitude value of the modulation signal applied to the optical modulation element departs from the predetermined value.
In addition, when the various type functional elements such as an amplifier, a distributor, and a phase shifter are mounted on the connection substrate, operation characteristics of them vary depending on temperature variation. For example, there is a case of variation in an amplitude value of the modulation signal output from the functional element. Consequently, the amplitude value of the modulation signal applied to the optical modulation element departs from a predetermined value, thereby causing deterioration in modulation characteristics of the optical modulator such as deterioration in S/N ratio and distortion in waveform.
In addition, in the connection substrate 4 mounted in the optical modulator module, when a microwave signal, which is the modulation signal, is guided into the substrate, a radiation mode 13 of the microwave signal occurs in the signal input end 11, thereby causing the effect that a part of the modulation signal is radiated inside the connection substrate, as shown in FIG. 1(b). Hence, the voltage amplitude value of the modulation signal is varied, and thus it is difficult to apply to the optical modulation element the modulation signal having the predetermined amplitude value. Moreover, when the amplifier is mounted on the functional element 8, the modulation signal itself inputted to the amplifier varies. As a result, the modulation signal output from the amplifier becomes larger and departs from the predetermined amplitude value.
Besides, when monitoring means (not shown in the drawings) for monitoring the voltage amplitude value of the modulation signal is provided outside the optical modulator module of FIG. 1, the modulation signal of which the voltage amplitude value is monitored is inputted to the inside of the optical modulator module through the GPO connector 7. Hence, when the voltage amplitude of the modulation signal is reduced by connection loss in the connector 7, the voltage amplitude value of the modulation signal applied to the optical modulation element 1 is remarkably different from the voltage amplitude value of the modulation signal monitored by the monitoring means, and thus it is difficult to monitor the voltage amplitude value of the modulation signal with high accuracy. As might be expected, it is also the same in a case where the amplifier of the modulation signal is provided outside the optical modulator module.